1. Field of the Invention
The present invention relates to an equalization process, and more particularly to a method of reducing a number of computations in an equalization process and an apparatus for performing the same.
2. Description of Related Art
Increasing consumer demand for mobile services is a driving force in developing mobile communication systems, such as the Global System for Mobile Communications (GSM).
Enhanced Data rates for GSM Evolution (EDGE), a faster version of the GSM wireless service, is designed to deliver data at rates up to 384 Kbps and enable the delivery of multimedia and other broadband applications to mobile phones and computer users. The EDGE standard is built on the existing GSM standard, using the same time division multiple access (TDMA) frame structure and existing cell arrangements.
To achieve higher data rates, the EDGE standard employs a new modulation scheme, known as 8-phase shift keying (8-PSK). Another modulation scheme, Gaussian minimum shift keying (GMSK) may also be used in situations where a more robust modulation is needed.
The GMSK modulation uses a signal space with the signal points +1 and −1, thereby enabling each pulse to carry 1 bit of information. The 8-PSK modulation uses a signal space with eight signal points, the phase difference between the individual signal points being 45 degrees, thereby enabling each pulse to carry 3 bits of information. As the amount of information carried by each pulse is higher, the 8-PSK modulation becomes more vulnerable to errors caused by intersymbol interference (ISI).
The ISI is the residual effect of other neighboring symbols when decoding a certain symbol, and this residual effect is due to the occurrence of pulses before and after the sampling instant.
The presence of the ISI in a mobile communication system, such as a system based on the EDGE standard, introduces errors in a receiver. Therefore, a filter design of an EDGE receiver and transmitter seeks to diminish the effects of the ISI and thereby deliver data to the destination with the smallest error rate possible.
FIG. 1 is a block diagram illustrating an equalization system for reducing ISI.
Referring to FIG. 1, the equalization system 100 includes a channel estimator 110, a prefilter 120 and a delayed decision feedback sequence estimator (DDFSE) 130.
The channel estimator 110 is needed in a system employing a coherent modulation and demodulation scheme, and in the EDGE standard, and receives a training sequence included in a burst to output an estimated channel model.
The prefilter 120 matches a channel so as to increase a signal-to-noise at) ratio (SNR) at a decision time when a certain symbol is determined. Typically, the prefilter 120 receives, from the channel estimator 110, coefficients for a prefiltering operation to transmit, to the DDFSE 130, complex symbol spaced samples.
The DDFSE 130 reduces the ISI of an input signal to output an estimated signal, and receives parameters from the channel estimator 110 and the prefilter 120 to output an ISI-reduced signal.
The operation of the equalization system 100 of FIG. 1 is described as follows.
In the receiver, the channel estimator 110 estimates a channel so as to reduce an ISI error and the prefilter 120 performs a prefiltering operation so as to perform an equalization operation. The DDFSE 130 receives, from the channel estimator 110 and the prefilter 120, parameters which are adjusted based on a channel characteristics.
However, in an equalization operation, the prefiltering operation performed by the prefilter 120 performs a number of computations. Therefore, a main processor needs to operate at a high clock frequency so as to perform a prefiltering operation and as a result, consumes a large amount of power.
In the EDGE standard, when the transmitter modulates a signal by use of the 8-PSK modulation scheme, the receiver typically demodulates the signal by use of a method of delayed decision feedback sequence estimation and is capable of performing sequence estimation after receiving a channel response without a precursor. The prefilter 120 changes the channel response to the sequence estimation and needs a large number of calculations for overall equalization system operation.